1. Field of the Invention
The present invention relates to a current limiter for cutting off a current path when a current flowing therethrough exceeds a predetermined limit.
2. Prior Art
A conventional current limiter as disclosed in U.S. Pat. No. 3,846,729 entitled "Current Limiter" issued on Nov. 5, 1974 comprises a bimetal having one end secured to a first fixed terminal, and a semielliptic spring having one end held in engagement with the other free end of the bimetal to urge the latter so as to be compressed longitudinally thereof. When the free end of the bimetal is located in a critical line extending between the other end of the semielliptic spring and the secured end of the bimetal, the free end of the bimetal is unstable and tends to get stabilized in position by moving to either side of the critical line. The current limiter also includes a second fixed terminal disposed at one of the stable positions of the bimetal and normally in contact with the free end of the bimetal. When a current which flows between the first and second stationary terminals via the bimetal is below a predetermined amount, the bimetal remains as it is without being bent. When the current exceeds such a predetermined value, then the bimetal becomes curled due to Joule's heat generated, enabling its free end to move across the critical line toward the other stable position away from the second fixed terminal. Thus, the current path between the first and second fixed terminals is cut off. The prior current limiter is advantageous in that the bimetal may be slightly springy, can be reduced in thickness, may be of a high resistance, and can be put into operation with a relatively small current. Even if the bimetal is of a relatively large thickness, it can be assisted by the semielliptic spring for rapid cut-off operation.
The semielliptic spring of the known current limiter, however, tends to fatigue during repetitive operation. Such a disadvantage is not negligible especially with current limiters of the automatic return type because when the bimetal is returned to its original position at a reduced temperature after it has cut off the current path, the bimetal may again be curled off soon by a current flowing therethrough which exceeds the predetermined amount. Thus, the bimetal is caused to make and break the circuit repeatedly, resulting in rapid fatigue of the semielliptical spring and malfunctioning of the current limiter.
With current limiters of a conventional design, the bimetal must have a greatly reduced resistance in order to be actuatable by a high current such as 20 (A) or more. Therefore, the bimetal must have an increased thickness resulting in an increased degree of stiffness thereof, with the result that the semielliptical spring is required to have increased resiliency. To meet such a requirement, the semielliptical spring should be made of a material having increased modulus of elasticity, such as stainless steel, with an increased thickness. As the thickness of the semielliptical spring is increased, the maximum stress thereof is reduced in inverse proportion, and the spring is the more liable to get bent, with the consequence that the semielliptical spring will have a much shorter service life. With the semielliptical spring being of an increased resiliency, it becomes difficult to assemble in place and cannot be reliably held in engagement with the bimetal. Where the semielliptical spring has a hole therein for engagement with the bimetal, the spring has a tendency to get deformed at the hole during assembly or operation.